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conditional sentences...........plz help

can u plz explain this with one more example to me,,,,,,,,,,,,,,explain me with an example that expalin that which line best suites for that particular situation,,,,,,,,it will be very help full for me.: like when i say this:
1...........If I knew, i would tell u ( why cannot say, that i donot know , so i cannot tell you, instead of using knew and would)

And in which situation i should use this line instead of the first one
2.............If I had known, I would have told you (why cannot say, i did not know so thats why i could not tell you------i know it means like this , then why cannot say in this way,,,,,,,,,,why should use had known, and would have)
3.............i got this point, from a tv show, it was like this:
-----i could have stopped her, but i failed to do so, EVEN IF I WOULD HAVE STOPPED HER, IT WOULD NOT HAVE HELPED.............
so i am confuse, coz always in the conditional sentences, line start from if,,,then next line with would,,,,,,or visa versa....like:
--- if i knew, i would tell you.
----if i had heared the weather report, i would have taken an umbrella etc.
soooooooooo, here howcome both lines are with would,,,,,,,,,,should not be like this:
--------even if i had stopped her, it would not have helped
----------even if i stopped her, it would not have helped
------------even if i stopped her, it would not helped,,,,,,,,
sorry, but this makes me veryyy confuse,,,,,,i want to clear this thing,,,plz help me,,,,,,,,i always get confused in conditional sentences, that where to use past simple,,,,and where to use past parfect in conditional sentences with the word would,,,,,,,,,,,,,,,,,,,,,,,